Fusing device with stable nipping portion

Abstract

In a device for fusing a toner image on a recording medium, a first roller provided with a heat source and a second roller are provided. A stretcher is elongated in a longitudinal direction of the second roller. An endless belt is stretched by an outer circumferential face of the second roller and a first face of the stretcher, and circulated in accordance with rotation of the first roller and the second roller. The endless belt is sandwiched between the first roller and the second roller to form a nipping portion through which the recording medium passes to fuse the toner image thereon. A supporter is disposed between the second roller and the stretcher. The supporter is elongated in the longitudinal direction of the second roller, and is provided with at least one first projection having a first face which is brought into slide contact with at least a longitudinal center portion of the outer circumferential face of the second roller.

1. A device for fusing a toner image on a recording medium, comprising:
 a first roller, provided with a heat source; 
 a second roller; 
 a stretcher, elongated in a longitudinal direction of the second roller; 
 an endless belt, stretched by an outer circumferential face of the second roller and a first face of the stretcher, and circulated in accordance with rotation of the first roller and the second roller, the endless belt being sandwiched between the first roller and the second roller to form a nipping portion through which the recording medium passes to fuse the toner image thereon; and 
 a supporter, disposed between the second roller and the stretcher, the supporter being elongated in the longitudinal direction of the second roller, and provided with at least one first projection having a first face which is brought into slide contact with at least a longitudinal center portion of the outer circumferential face of the second roller. 
 
   
   
     2. The fusing device as set forth in  claim 1 , wherein the stretcher is provided with at least one second projection having a second face which is brought into contact with the supporter so as to oppose to the first projection. 
   
   
     3. The fusing device as set forth in  claim 1 , further comprising:
 a pair of arm members, each of which has a first end which is pivotably fitted with a rotary shaft of the second roller and a second end which is coupled with one of longitudinal ends of the stretcher, so that the stretcher is retractably brought into contact with the first roller through the belt; and 
 a coiled spring, interposed between the one of the longitudinal ends of the stretcher and the second end of the arm members so as to urge the stretcher in a direction separating from the second roller. 
 
   
   
     4. The fusing device as set forth in  claim 3 , wherein an axial line of the coiled spring passes through a position which is shifted from a rotary axis of the second roller in a direction separating from the first roller. 
   
   
     5. The fusing device as set forth in  claim 3 , wherein both longitudinal ends of the supporter are pivotably fitted with the rotary shaft of the second roller. 
   
   
     6. The fusing device as set forth in  claim 3 , wherein both longitudinal ends of the supporter are formed with flanges which are respectively fitted with both longitudinal ends of the second roller. 
   
   
     7. The fusing device as set forth in  claim 1 , wherein the first face of the first projection and the outer circumferential face of the second roller have an identical curvature. 
   
   
     8. The fusing device as set forth in  claim 2 , the second face of the second projection and a face of the supporter contacting the second projection have an identical curvature. 
   
   
     9. The fusing device as set forth in  claim 1 , wherein a portion of the stretcher which is to be brought into contact with the first roller through the belt is made flat. 
   
   
     10. The fusing device as set forth in  claim 1 , wherein the supporter is provided with a projection, and the stretcher is provided with a hole into which the projection is fitted. 
   
   
     11. The fusing device as set forth in  claim 1 , further comprising an urging member, which urges the stretcher toward the first roller. 
   
   
     12. The fusing device as set forth in  claim 1 , wherein a longitudinal center portion of the first face of the stretcher has a smaller curvature than both longitudinal ends thereof. 
   
   
     13. The fusing device as set forth in  claim 1 , further comprising a plurality of coiled springs, disposed between the stretcher and the supporter and arrayed in the longitudinal direction of the second roller. 
   
   
     14. The fusing device as set forth in  claim 1 , wherein:
 the stretcher is disposed at an upstream side, in the circulating direction of the belt, of a contact position at which the second roller contacts the first roller through the belt; and 
 the belt is wrapped around an outer circumferential face of the first roller by the stretcher to form the nipping portion. 
 
   
   
     15. The fusing device as set forth in  claim 1 , wherein:
 the stretcher is disposed at a downstream side, in the circulating direction of the belt, of a contact position at which the second roller contacts the first roller through the belt; and 
 the belt is wrapped around an outer circumferential face of the first roller by the stretcher to form the nipping portion. 
 
   
   
     16. The fusing device as set forth in  claim 13 , wherein the first projection is provided so as to oppose to each of the coiled springs. 
   
   
     17. The fusing device as set forth in  claim 13 , wherein an axial line of each of the coiled springs passes through a position which is shifted from a rotary axis of the second roller in a direction separating from the first roller. 
   
   
     18. The fusing device as set forth in  claim 13 , wherein at least two of the springs have different urging forces. 
   
   
     19. The fusing device as set forth in  claim 18 , wherein an urging force of one of the springs disposed in a longitudinal center portion of the stretcher is greater than an urging force of another one of the springs disposed in a longitudinal end portion of the stretcher. 
   
   
     20. The fusing device as set forth in  claim 18 , wherein an urging force of one of the springs disposed in a longitudinal center portion of the stretcher is smaller than an urging force of another one of the springs disposed in a longitudinal end portion of the stretcher. 
   
   
     21. The fusing device as set forth in  claim 18 , wherein an urging force of two of the springs disposed in a longitudinal center portion and a longitudinal end portion of the stretcher is greater than an urging force of another one of the springs disposed in a position between the longitudinal center portion and the longitudinal end portion. 
   
   
     22. The fusing device as set forth in  claim 1 , wherein the supporter has a portion operable to press the belt against the first roller. 
   
   
     23. The fusing device as set forth in  claim 1 , wherein a plurality of the first projections are disposed so as to come in contact with separate positions of the outer circumferential face of the second roller in the longitudinal direction thereof. 
   
   
     24. The fusing device as set forth in  claim 1 , wherein a plurality of the first projections are disposed so as to come in contact with separate positions of the outer circumferential face of the second roller in a rotating direction thereof. 
   
   
     25. The fusing device as set forth in  claim 24 , wherein the first projections are provided as roller members. 
   
   
     26. The fusing device as set forth in  claim 1 , further comprising:
 a pair of arm members, each of which has a first end which is pivotably fitted with a shaft member and a second end which is coupled with one of longitudinal ends of the stretcher, so that the stretcher is retractably brought into contact with the first roller through the belt; and 
 a coiled spring, interposed between the one of the longitudinal ends of the stretcher and the second end of the arm members so as to urge the stretcher in a direction separating from the second roller, 
 wherein the shaft member is disposed in the vicinity of a rotary shaft of the second roller. 
 
   
   
     27. An image forming apparatus incorporating the fusing device as set forth in  claim 1 , comprising:
 an image former, which forms the toner image on the recording medium; and 
 a transporter, which transports the recording medium having the toner image to the fusing device.